1. Field of the invention
The present invention relates to a control device for an antiglare mirror for use in vehicles.
2. Description of Related Art
Antiglare mirrors for use in vehicles using a gel-type EC cell and a thin-film type EC cell have been conventionally proposed. The gel-type EC cell, which is formed of gel material such as biologen compound sealed between glass substrates, uses coloring due to oxidation/reduction of the biologen compound when a voltage is applied to the glass substrates. The thin-film type EC cell, which is formed of a transient metal oxide such as WO.sub.3 vapor-deposited on the glass substrates, uses coloring due to injection of hydrogen and metal ions into and emission thereof when a voltage is applied to the glass substrates. Therefore, whether the antiglare mirror uses the gel-type EC or the thin-film type EC cell, as the applied voltage becomes high, its mirror reflectivity decreases due to the coloring of the EC cell, and when the applied voltage is removed, it is restored to its initial value and thereafter increases.
The gel-type EC, which has a simple structure in which the gel material is sealed between two glass substrates, is high in productivity. But it is not suitable to a spherical mirror which must be formed by bending the glass substrate. For this reason, actually, only a plane mirror has been formed using the gel type EC cell. On the other hand, although the thin-film type EC cell, which has the structure of vapor-deposition to the glass substrates, requires great initial investment for e.g. a vapor-deposition apparatus, it is suitable to fabrication of the spherical mirror as well as the plane mirror. Thus, where both inner mirror and outer mirror are plane mirrors, it is desirable to fabricate all the mirrors using the gel type EC cell. But in a general case where the outer mirror is a spherical mirror, it is necessary to use the gel type EC cell as an inner mirror and the thin-film type EC cell as an outer mirror.
However, the gel-type EC cell is a system of controlling its mirror reflectivity by application of a positive voltage whereas the thin-film type EC cell is a system of controlling its mirror reflectivity by application of both positive and negative voltages. Therefore, when the gel-type EC cell is used as the inner mirror and the thin-film EC cell is used as the outer mirror, it is impossible to control the mirror reflectivity of the outer mirror by a driving circuit for the inner mirror. In this case, the mirror reflectivity of the outer mirror must be controlled separately from the control for the inner mirror. As a result, the entire structure becomes complicated increasing production cost, and the antiglare states of the inner mirror and outer mirror may not be synchronized with each other.